Telephone credit or calling cards, although convenient, are susceptible to being compromised by potential unauthorized users. Indeed, fraudulent use of such cards has become a problem. To combat such fraudulent use, telecommunications systems may employ automatic speech recognition to verify the identity of a caller who is charging a call to a credit or calling card. Typically, such verification is based on the caller uttering an assigned subscriber identifier. The assigned identifier may be composed of a number of digits, for example, seven digits plus a number of so-called check digits, e.g., two check digits, and is typically used to index indirectly an associated subscriber record stored in memory. It can be appreciated that if such a fraud prevention scheme proves to be popular with telephone callers, then the number of callers who subscribe to the service could exceed the number of available identifiers. This situation may be addressed by increasing the number of digits forming an identifier, thereby increasing the number of available identifiers that may be assigned to subscribers. Herein lies the nub of the problem. On the one hand, the number of digits forming an identifier should not be large, since subscribers may find it difficult to remember their respective identifiers. On the other hand, the number of digits forming an identifier may have to be increased to accommodate a large number of subscribers.